1. Field of the Invention
The present invention relates to a photothermographic material. More particularly, the invention relates to a photothermographic material which has high image quality and is appliciable for recording images for medical diagnosis.
2. Description of the Related Art
In recent years, it has been strongly desired in the field of films for medical imaging to reduce the amount of used processing liquid waste in consideration of environmental protection and space saving. For this reason, technology regarding photothermographic materials as films for medical diagnosis and for photographic applications, which are capable of efficient exposure with a laser image setter or a laser imager and capable of forming a clear black-toned image with high resolution and high sharpness is desired. Such photothermographic materials can eliminate use of liquid processing chemicals and can provide users with a thermal development system which is simpler and does not contaminate the environment.
Although similar requirements also exist in the field of general image forming materials, an image for medical imaging requires a particularly, high image quality excellent in sharpness and granularity because a delicate image representation is necessitated. Also an image of blue-black tone is preferred in consideration of easy diagnosis. Currently, various hard copy systems utilizing pigments or dyes, such as ink jet printers and electrophotographic systems, are available as general image forming systems, but they are not satisfactory as output systems for medical images.
On the other hand, thermal image forming systems utilizing organic silver salts are described in many documents. Specifically, a photothermographic material generally comprises an image forming layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt) and, if necessary, a toner for controlling the tone of a developed silver image are dispersed in a matrix of a binder. The photothermographic material, when heated at high temperature (for example, 80° C. or higher) after image exposure, forms a black-toned silver image by an oxidation/reduction reaction between the silver halide or the reducible silver salt (functioning as an oxidizing agent) and the reducing agent. The oxidation/reduction reaction is promoted by a catalytic effect of a latent image formed by exposure on silver halide. As a result, a black silver image is formed in an exposed region. Further, Fuji Medical Dry Imager FM-DP L is an example of a practical medical image forming system using a photothermographic material that has been marketed.
Organic halogen compounds are known as antifoggants for photothermographic materials. For example, Japanese Patent Application Laid-Open (JP-A) Nos. 2000-284410, 2003-186139 and 2001-312027 disclose organic polyhalogen compounds having specified substituents. Further, JP-A No. 2000-284409 discloses preventing black spots, which are a problem peculiar to such an image forming system, from emerging by employing different organic polyhalogen compounds in combination in an ultra-high contrast photothermographic material using a nucleator for use in graphic arts.